search: add panic for unsupported job case in printer
Created by: rvantonder
Stacked on https://github.com/sourcegraph/sourcegraph/pull/31205
Follow up of https://github.com/sourcegraph/sourcegraph/pull/31199#discussion_r806230157 where I was too eager to merge
Test plan
Semantics-preserving, just makes things fail hard instead of silently for debugging.